The present invention relates to individually protected strands used in civil engineering structures, especially for prestressing or suspending structure portions.
These strands comprise an assembly of metal wires twisted together, which are usually seven in number. The metal wires are frequently subjected to an electrochemical treatment (galvanizing, galfanizing, etc.) providing a certain corrosion resistance.
It is common practice to use uncoated strands, taking care to ensure that they are not placed in a corrosive environment. These strands are placed directly in concrete or within collective sheaths filled with cement grout or with petroleum waxes or greases. The passivity of the cement or the non-corrosivity of the petroleum-based products enhances the corrosion protection.
Strands are also known which are individually protected by a plastic sheath, usually made of a high-density polyethylene (HDPE) or an epoxy, which creates an sealed barrier around the metal wires. A filling compound, which may be of several types (wax, grease, polymer, etc.), fills the gaps existing between the metal wires and the individual sheath in order to enhance the corrosion protection of the strand.
The filling compound allows either slip of the stranded metal wires with respect to their individual sheath (greased-sheathed or waxed-sheathed strand) or, on the contrary, adhesion in order to transmit shear forces between the sheath and the strand (bonded strand).
In the bonded strand, the filling compound is typically a polymer adhering to the wires and to the inside of the sheath. Bonded strands can especially be used when it is necessary to transmit shear forces from the sheath to the metal wires, this being the case for example in the cables supporting suspension bridges whereby the load transmitted by each hanger creates a tangential force on the cable at the clamp where the hanger is fastened (see EP-A-0 855 471).
In the greased-sheathed or waxed-sheathed strand, the filling compound is a lubricant. This has several advantages:
(a) it improves the fatigue behaviour of the strand by lubricating the contacts between its metal wires;
(b) it prevents the tension to which the strand is subjected from generating, due to the shape of the strand, shear and/or tensile stress concentrations in certain portions of the strand, which may cause the sheath to crack, and therefore to no longer seal, exposing the metal to corrosive agents;
(c) in certain configurations, it allows the strands to be replaced one by one, the sheath remaining in place in the structure.
In service, a cable comprising one or more greased-sheathed or waxed-sheathed strands is subjected to tension variations and to temperature variations. These variations cause different elongations of the sheath and of the stranded wires since the plastic and the metal generally do not have the same elasticity and thermal expansion coefficients.
In particular, the sheath usually has a much higher thermal expansion coefficient that the wires. If we consider the case of steel and HDPE, widely used in this kind of strand, the ratio of the two thermal expansion coefficients is of the order of 20. This may result in damage to the sheath, which elongates too much when it is hot or, conversely, a loss of sealing in the end portions of the cable when it is cold, the sheath contracting too much.
An object of the present invention is to avoid these drawbacks, while maintaining at least some of the advantages of the greased-sheathed or waxed-sheathed strand.
A strand according to the invention comprises a group of twisted metal wires, a plastic sheath containing said group, and a pliant filling compound which fills internal interstices lying between the twisted wires of the group and a peripheral interstice lying between the periphery of the group and the inner face of the sheath. According to the invention, said peripheral interstice has, in a cross section of the strand, an area of between Pxc3x97emin and 0.6xc3x97S2, where P is the external perimeter of the group of wires, emin=0.05 mm and S2 is the cumulative area of the gaps lying between the periphery of the group and the smallest circle within which the group is inscribed.
It is thus possible to obtain xe2x80x9csemi-adherentxe2x80x9d strands in which the regulated amount of pliant filling compound makes it possible to retain the advantages (a) and (b) of the greased-sheathed strand while still ensuring that the individual sheath follows the macroscopic deformations of the metal wires.
The helical ribs present in the inner face of the sheath penetrate the grooves formed between the adjacent peripheral wires. Cooperation between these ribs and these grooves allows matching of the macroscopic deformations. The amount of filling compound is adjusted so that this penetration is not too great, which might cause locking of the sheath onto the wires by shape adhesion and hence generate stresses in the sheath, especially shear stresses, liable to tear it.
In a preferred embodiment of the invention, the sheath of the strand has a thickness of at least xcfx86/5, where xcfx86 is the diameter of the wires lying at the periphery of the group of twisted wires.
Another aspect of the invention relates to the use of a strand as defined above as structural element working in tension in a building structure. In particular, the strand may form part of a stay cable of a suspension system for the structure, or of a pre-stresing cable for the structure.
A third aspect of the invention relates to a process for manufacturing a strand, comprising the steps of:
coating a group of twisted metal wires with a pliant filling compound so that said compound fills internal interstices lying between the twisted wires of the group and protrudes at the periphery of the group;
wiping the periphery of the coated group so as to leave a regulated amount of filling compound per unit length of the group, said amount representing a volume per unit length of between S1+(Pxc3x97emin) and S1+(0.6xc3x97S2), where S1 is the cumulative area of said internal interstices on a cross section of the strand, P is the external perimeter of the group of twisted wires, emin=0.05 mm and S2 is the cumulative area of the gaps lying between the periphery of the group and the smallest circle within which the group is inscribed;
extruding a plastic sheath around the group of wires coated with said amount of filling compound.
The wiping step is advantageously carried out by means of a pivotally mounted template, through which the coated group of wires is fed.